This invention is related to polariscopes, and more particularly to polariscopes that are portable.
A polariscope shows and qualitatively measures stress in glass and other photoelastic materials. When isotropic, photoelastic materials, such as glass and some plastics, are stressed, they become anisotropic. The anisotropy within the material makes the stressed areas birefringent. With a polariscope, one can determine the degree of anisotropy by observing the degree of birefringence, and determine a relative direction of tensile and compressive stress vectors. These can then be related qualitatively to the amount of stress within the material. A polariscope generally includes a light source, a first polarizer, a full wave plate and a second polarizer. The article to be tested is placed between the full wave plate and the second polarizer.
The above-cited application proposes an LED light source for a polarimeter instead of the commonly-used incandescent lamp. Although the incandescent lamp produces a smooth spectrum of white light, nearly 85% of the energy input to the lamp is converted to heat. This excessive heat causes damages to the polarizing film of the first polarizer that is adjacent to the lamp in the polarimeter. Over a relatively short time period, the damage to the film diminishes the ability of the polarimeter to accurately measure stress.
The LED light source in the above-cited application can also replace the compact fluorescent lamps that have been used in polarimeters to diminish the damage to the polarizing film from heat. However, the use of fluorescent lamps greatly increases the chance for chromatic aberrations. A failed lamp must be replaced with a reasonably exact duplicate otherwise the difference in the emission spectrum will shift the calibration of the polarimeter. In the case of fluorescent lamps, it is easy to make a mistake since the emission spectrum of a fluorescent lamp is a function of the phosphors used in its manufacture, e.g., there are several “colors” of white for commercial fluorescent lamps. Likely, the operator will be unaware of the color shift of the lamp and therefore not be able to compensate for it.
The manufacture of glass articles includes the controlled cooling of the glass to avoid generating stresses in the glass. A temperature difference within the glass as it cools through the glass transition range can cause stress in the glass. The intensity of the stress is directly related to the contraction properties of the glass and the intensity of the temperature difference. A polariscope can be used in a glass article manufacturing line to determine where alterations in the cooling process are needed to equalize the temperatures in the glass. Specifically, the hot glass article is removed from the line, typically while the line is in motion, and the glass is placed in the field of view of a polariscope. At some point in time, the glass article will exhibit birefringence (stress) indicating that the article is passing through the glass transition range. The time when the stress first appears must be measured accurately so the relative position of the manufacturing line can be determined. This operation can be dangerous for the operator and tends to be less accurate than desired.
Heretofore, the use of an incandescent or fluorescent lamp polariscope on a glass article manufacturing line without removing the glass article from the line has not been practical because of the size and weight of the polariscope. A typical “portable” polariscope is 8″ in diameter, 14″ high, weighs 11 pounds, is connected to a cumbersome electrical cable, and has a light source that is about 10″ deep. In use, the light source should be behind the glass article so that a reading at the second polarizer can be made. However, the clearance behind the glass article on a typical manufacturing line is usually only 2–4″ (the minimum required for the motion of mechanical devices and the location of burners), far too narrow for the 10-inch depth of the light source of the available polariscopes.
It would be an advantage to have a polariscope that is usable on a glass article manufacturing line without removing the glass article and that overcomes the above-noted difficulties.